The present invention generally relates to the measurement of properties of an incoming air stream, and more particularly, to the measurement of velocity and incident angle of an incoming air stream.
The measurement of the velocity and incident angle of an incoming air stream has many applications, including flight control applications, industrial process stream applications, combustion control, weather monitoring applications, etc. For flight control applications, the precise orientation or attitude of an aircraft relative to an incoming air stream, as well as the air velocity, are important components of the total xe2x80x9cair dataxe2x80x9d information package used by modem flight control systems.
In most cases, air speed is detected by sensing the difference between head and static pressure, often using Pitot tubes. This approach operates well at speeds above about 60 knots if a very accurate differential pressure sensor or two absolute pressure sensors are used, at considerable expense. Additional sensors are typically needed to detect the orientation or attitude of an aircraft.
One way to detect the attitude of an aircraft is to use vane transducers, which include several mechanically rotating vanes that find an orientation that results in a balanced pressure or air speed on either of side of the vanes. By detecting the resulting orientation of the vanes, the attitude of the aircraft can be determined. A limitation of such a sensor system is that the mechanical rotating vanes often reduce the reliability and response time of the sensor. In addition, most vane transducers have a relatively large radar cross-section, which is undesirable in many applications, such as military applications.
The present invention overcomes many of the disadvantages of the prior art by providing a rugged microsensor assembly that can measure both velocity and angular direction of an incoming air stream. The microsensor assembly preferably includes at least two flow sensors, each orientated to measure a different velocity component of the incoming air stream. The velocity components are related by the geometry between the sensors. The angular direction and velocity of the incoming air stream can be determined by examining the measured velocity components. Such a microsensor can provide a fast response time and a relatively small radar cross-section It is contemplated that the microsensor of the present invention may be used in a wide variety of applications, including for example, flight control applications, industrial applications, weather monitoring applications, etc.
In one illustrative embodiment of the present invention, the sensor assembly includes a first sensor and a second sensor. The first sensor measures the velocity component of the incoming air stream that extends along a first sensor axis. The second sensor measures the velocity component of the incoming air stream that extends along a second sensor axis, wherein the first sensor axis is rotated from the second sensor axis to intersect the second sensor axis at an intersection point.
From the outputs of the first and second sensors, the angular direction and velocity of the incoming air stream can be determined. During operation, if the angular direction of the air stream deviates in one direction, the velocity components of the incoming air stream that extend along the first sensor axis increase, and the velocity components of the incoming air stream that extend along the second sensor axis decrease. Likewise, if the angular direction of the air stream deviates in the other direction, the velocity components of the incoming air stream that extend along the first sensor axis decrease, and the velocity components of the incoming air stream that extend along the second sensor axis increase. By examining the velocity components measured by the first and second sensors, and using the relative geometry between the sensors, the angular direction of the air stream can be determined.
Preferably, both the first sensor and the second sensor are thin-film microanemometers such as available microbridge flow type sensors, each having at least one elongated heater element and at least one elongated sensor element, both in thermal communication with the incoming air stream. The elongated heater and sensor elements preferably extend perpendicular to the associated sensor axis. For example, the elongated heater and sensor elements of the first microbridge flow sensor preferably extend perpendicular to the first sensor axis, and the elongated heater and sensor elements of the second microbridge flow sensor preferably extend perpendicular to the second sensor axis.
The heater elements of the first and second microbridge flow sensors are then energized by either a common or separate heater energizers. The heater energizers preferably cause an elevated temperature condition in each of the elongated heater elements, which in turn, cause an elevated temperature condition in adjacent upstream and downstream sensor elements, and in the air stream. The temperature distribution near the thin-film bridge is transmitted symmetrical about the heater element when no air flow is present, and is disturbed when air flow is present. The amount of disturbance is related to the velocity of the air-stream along the corresponding sensor axis.
The sensor elements of the first and second microbridge flow sensors preferably have a resistance that changes with temperature. Accordingly, the sensor elements of the first microbridge flow sensor can be used to sense the temperature distribution provided by the heater element of the first microbridge flow sensor. Likewise, the sensor elements of the second microbridge flow sensor can be used to sense the temperature distribution provided by the heater element of the second microbridge flow sensor.
More specifically, and in one illustrative embodiment, one sensor element is positioned upstream from the heater element, and the other is positioned downstream. The heater element is then heated a predetermined amount above the ambient temperature of air-stream. When there is a positive air-stream, the upstream sensor element is cooled, and heat conduction from the heater element to the downstream sensor element is promoted. As a result, the temperature of the downstream sensor element is increased, and a difference in temperature between the sensor elements appears. This temperature difference can be related to the velocity component of the air-stream along the corresponding sensor axis.
Alternatively, and in another illustrative embodiment, the heater energizer provides a transient elevated temperature condition in each of the elongated heater elements, which in turn, causes a transient elevated temperature condition in the air stream. Each sensor element, which preferably has a resistance that changes with temperature, can be used to sense when the transient elevated temperature condition in the air stream arrives at the corresponding sensor element. The time lag between the transient elevated temperature condition in the heater element and the resulting transient elevated temperature condition in the sensor elements can be related to the velocity component of the air-stream along the corresponding sensor axis.
In this embodiment, each microbridge flow sensor may have a corresponding time lag detector for determining the time lag values. One time lag value may correspond to the time lag, or delay, between the transient elevated temperature condition in the heater element and the resulting transient elevated temperature condition in a first (e.g., upstream) sensor element. Another time lag value may correspond to the time lag between the transient elevated temperature condition in the heater element and the resulting transient elevated temperature condition in a second (e.g., downstream) sensor element.
The velocity component of the incoming air stream that extends along the first sensor axis can be determined using the two time lag values of the first microbridge flow sensor. Likewise, the velocity component of the incoming air stream that extends along the second sensor axis can then be determined using the two time lag values of the second microbridge flow sensor. Other illustrative methods and sensor configurations for determining the velocity component of the incoming air stream along the corresponding sensor axis are disclosed in U.S. Pat. Nos. 4,478,076, 4,478,077, 4,501,144, 4,651,564, 4,683,159, 5,050,429, U.S. patent application Ser. No. 09/002,157, filed Dec. 31, 1997, entitled xe2x80x9cTime Lag Approach For Measuring Fluid Velocityxe2x80x9d, U.S. patent application Ser. No. 09/001,735, filed Dec. 31, 1997, entitled xe2x80x9cSelf-Oscillating Fluid Sensorxe2x80x9d, and U.S. patent application Ser. No. 09/001,453, filed Dec. 31, 1997, entitled xe2x80x9cFluid Property and Flow Sensing Via a Common Frequency Generator and FFTxe2x80x9d, all of which are assigned to the assignee of the present invention and incorporated herein by reference.
The heater energizers are preferably active circuits with feedback that provide whatever power or voltage is necessary to the heater elements to maintain a heater temperature that is a fixed amount above the ambient temperature of the incoming air-stream This helps maintain an adequate signal to noise ratio for each of the microbridge sensors.
To increase the reliability and accuracy of the sensor, the power or voltage that is applied to the heater elements may be monitored. The heater power or voltage can then be used to provide a redundant signal for air-stream velocity. As the velocity of the air-stream increases, the amount of power or voltage required to maintain the heater temperature at the fixed amount above the ambient temperature of the air-stream also increases. Thus, there is a relationship between the voltage or power applied to the heater, and the velocity of the air-stream. The relationship is relatively independent of the direction of the incoming air-stream. Once the velocity of the air-stream is determined using the power or voltage signal of the heater element, it can be compared to the velocity determined using the sensor elements. If there is a substantial difference, an error flag may be set.
In addition, the power or voltage that is supplied to each heater element can be used to detect a change in the heat transfer load of the heater elements. Such a change can be caused by, for example, the presence of rain, sleet, ice, dust, or any other foreign material or substance on the sensor. When the heat transfer load changes, it is contemplated that a flow rate correction factor can be computed to compensate for the change in the heat transfer load. Alternatively, the sensor apparatus can be disabled until the heat transfer load returns to an expected range.
To help improve the ruggedness and reduce the frailty of conventional microbridge flow sensors, it is contemplated that the thickness of the bridge and/or a protective coating may be applied to the bridge. The thickness of the bridge of a conventional microbridge flow sensor is often on the order of 1 micron. In order to ruggedize such a sensor, it is contemplated that the thickness of the bridge may be increased to 15 microns or more. In one embodiment, the thickness of the xe2x80x9cbridgexe2x80x9d is increased to about 10 microns. As the thickness of the xe2x80x9cbridgexe2x80x9d increases, both the frailty and the response time of the sensor decrease, the output signal decreases but the S/N (signal-to-noise) ratio stays substantially the same. Thus, there is a balance between the degree of ruggedness, response time and sensitivity of the sensor. In order to sense high mass flow fluids, without reaching the saturation signal of thermal anemometers, it is desirable to xe2x80x9cdesensitizexe2x80x9d the flow sensor or effectively move the range of measurable flow velocities or mass fluxes to higher values.
Another approach for increasing the ruggedness as well as the ability to sense high mass flows of conventional microbridge flow sensors is to at least partially fill the cavity of a conventional microbridge flow sensor with a filler. The filler prevents the air-stream from flowing around both sides of the heater and sensor elements, and increases the thermal contact between the heater and sensor element, both of which reduce the sensitivity of the flow sensor. The filler also provides support to the bridge, and therefore leads to a more rugged structure than a conventional microbridge type sensor.
The filler preferably has a thermal coefficient of expansion that is substantially similar to the thermal coefficient of expansion of the material of the substrate (often silicon). The filler also preferably is a poor thermal conductor. In one embodiment, the filler is a UV curable epoxy. It is contemplated that the filler may assume a honeycomb, ribbed or embossed configuration, if desired.
Another approach for increasing the ruggedness and reducing the sensitivity of conventional microbridge flow sensors is to form the heater and sensor elements directly on a substrate (e.g. Pyrex glass). This eliminates the cavity and bridge of a conventional microbridge type sensor, and is referred to as a Microbrick(trademark) type flow sensor. Because the incoming air-stream does not flow around both sides of the heater and sensor elements, the sensitivity is reduced. Also, because the backsides of both the heater and sensor elements are supported by the substrate, the sensor is more rugged than a conventional microbridge type sensor.
Finally, it is contemplated that the thin contact wires used to connect the elements of a conventional microbridge flow sensor to off-chip components may be replaced with Through-The-Wafer (TTW) contacts. This increases the reliability of sensor because no fragile and electrically conductive wires or pads need to be exposed to possibly conductive fluid contaminates or fluid flow shear forces.